1. Field of the Invention
This invention pertains to a laminated glass with an electroconductive layer for automobiles or vehicles in general and more particularly windshields.
2. Background of the Prior Art
Glazings, generally made of two glass sheets combined with a transparent plastic interlayer of the polyvinylbutyral (PVB) type are well known. However, the electroconductive layer is almost always set on the internal side of the outer glass sheet, i.e. side 2, the sides of a bilayered laminated glass being conventionally numbered 1,2,3,4, starting from the outside of the glazing, that is to say the outside part when this glazing is mounted in a frame enclosing a vehicle, towards the inside.
Thus, the patent document EP 108616 of Donnelly Mirrors proposes a laminated glazing with two combined glass sheets having on side 2 an electroconductive layer of tin indium oxide (ITO) deposited by a vacuum technique.
The patent document U.S. Pat. No. 4,654,067 of Ford also proposes a similar type of windshield with the electroconductive layer located on side 2.
The same choice of side 2 is made in U.S. Pat. No. 2,944,296.
Other patent documents confirm the recurring choice of side 2 as a support for the electroconductive layer.
However, the patent document EP 192009 proposes to set the electroconductive layer, an ITO layer, on an outside part of the laminated glazing, i.e. side 1 or 4, in order to enable the thermal reduction treatment of the layer with a burner flame, a burner which would heat the ITO layer only, and not the glass and/or the PVB interlayer.
Apart from the last example, the recurring positioning of the layer on side 2 may be justified by the necessity to bend the glass after it has been coated. Moreover, in order to minimize the cracks and more generally the deterioration of the layer during the bending process, it should preferably be located on the concave rather than convex side of the glass. Otherwise, the layer would be put under extension, therefore increasing its risks of deterioration.
As an additional reason for selection of the recurring choice of side 2 for the electroconductive layer, one can point to the fact that it is extremely difficult to set a homogeneous layer by means of cathodic pulverization, with a plane cathode on a convex shape where the sides are in "shadow", rather than on a concave side, since side 2 on a windshield and a car in general is usually concave.
Moreover, in order to explain the recurring choice of side 2 for the electroconductive layer, one may have sought to facilitate the defrosting of a windshield by setting the said heating layer as close as possible to the surface to be defrosted.
However, if defrosting can be improved by positioning of the layer on side 2, this is not the case for defogging which is connected with side 4. Whereas defrosting only takes place at the starting of the car, standstill, or for a short period of time of about 3 min, and this only during certain days in wintertime, defogging is necessary all year long and most frequently when the car is running. As a consequence, defogging is not a minor function that can be overlooked. Furthermore, the defogging of a windshield internal side is generally difficult since the heating layer is located further away from the internal side to be defogged. This operation becomes even more difficult since the heating effects of the layer are decreased by the air streams on the windshield, i.e. the convective exchanges on the outside of the windshield increasing with the speed of the car.
Therefore defogging is very inefficient, if present at all when the vehicle is running and when the heating layer of the windshield is located on side 2. Even with a nonfogging windshield having a heating layer on side 2, additional means of defogging by air blowing (possibly not) have to be installed, which are both expensive and uncomfortable.
Furthermore, in case of breakage of the windshield by a stone, most of the time, only the external glass sheet of the windshield is damaged. Therefore, the conductive layer will also be damaged if located on side 2, that is to say the internal side of the broken glass sheet. The repairing of the windshield without dismantling, and be simply filling up the crack of the outside glass with a material similar to the glass index, will not repair the conductive layer. Most of the time, the deteriorated layer will still conduct electricity, but only in restricted areas and with a stronger intensity in these particular areas. This will lead to localized superheatings that may destroy the PVB plastic interlayer of the laminated glazing, making it non transparent locally and creating vision-disturbing defects incompatible with security.